The possibility of using liquid-phase instead of vapor-phase oxidation for the preparation of benzene carboxylic acids was first indicated by the disclosure in U.S. Pat. No. 2,245,528 of the catalysis provided by transition or variable-valence metals, especially cobalt, in a liquid phase of saturated lower aliphatic acid at temperatures from 100.degree. to 320.degree. C. and pressures to maintain the liquid phase of the aliphatic acid. Such catalysis, according to said patent, was advantageously promoted by the use of a ketone, such as methylethyl ketone, or aldehyde, such as acetaldehyde. Unfortunately, such aldehyde- or ketone-promoted variable-valence metal catalysis was useful only for converting mono-, di-, or trimethylbenzenes to their respective benzene monocarboxylic acids: benzoic, toluic, and dimethylbenzoic acids. Two separate, later, and somewhat parallel lower temperature (80.degree.-100.degree. C.) modifications of the aldehyde- or ketone-promoted cobalt catalysis in a liquid phase of acetic acid did provide commercially feasible conversion of xylenes to phthalic acids, especially p-xylene to terephthalic acid, but only at the expense of using rather high molar concentrations of cobalt and, with respect to p-xylene, rather large quantities of acetaldehyde or methylethyl ketone promoter which were oxidized to acetic acid.
The disadvantages of using high concentrations of cobalt promoted with large quantities of aldehyde or ketone are overcome by my novel process wherein molybdenum is used to activate the cobalt. According to my novel process, the use of molybdenum, cobalt, manganese, and bromine in a ratio of 0.001:1:1:2 to about 0.5:1:1:2 is effective in converting di- or polymethylbenzenes to their corresponding aromatic acids.
For the liquid-phase oxidation of di- and trimethylbenzenes with molecular oxygen, it has been discovered that molybdenum is particularly useful for substantially enhancing the activity of cobalt in a system where cobalt, manganese, and bromine are used as catalysts.
Cobalt is the most expensive component in a cobalt-manganese-bromine catalyst system, approximately ten to fifteen times more expensive than manganese. Therefore, there is great economic incentive to replace or reduce the cobalt component in the oxidation catalyst. My novel process has succeeded in doing just that by activating the cobalt moiety with molybdenum, thus reducing the total amount of cobalt to be used in a polyalkyl oxidation system wherein cobalt is the sole or most active component. A novel feature of molybdenum as a cobalt catalyst activator in the oxidation of polymethylbenzenes to the corresponding polycarboxylic acids is that much less cobalt is required to obtain satisfactory yields. However, the ratio of molybdenum to cobalt has to be in the range of about 0.005 to about 0.5 parts by weight. However, molybdenum is a catalyst poison when the ratio of molybdenum to cobalt exceeds about 0.5 parts by weight. Molybdenum is an effective promoter for the cobalt-manganese-bromine catalyst systems for the oxidation of polymethylbenzenes to the corresponding polycarboxylic acids. The term "activation", as used herein, means the ability of a catalyst component to increase the rate of oxidation of polymethylbenzenes to the corresponding polycarboxylic acids.
The data in Tables 2 through 7 are from a specially designed oxidation reactor in which specific effects on the rate of oxidation can be determined. The reactor operates at a temperature of 95.degree. C. and at atmospheric pressure. At this temperature and pressure the oxidation of alkylaromatic compound is occurring so slowly that the composition of the species being oxidized is essentially constant over the time period of the experiment. Because the composition of the alkylaromatic compound remains constant, so does the rate of oxidation. The effect of water on the rate of oxidation can now be evaluated by adding an increment of water to the reactor. For example, in Table 2 in Example 6, water is added to the reactor. This results in a decrease in the rate of oxidation from 6.66 to 5.64 to 4.33 to 1.43 ml oxygen reacted/min as increasing amounts of water are added. The addition of trimellitic acid has a similar effect (see Example 1). This demonstrates that both water and trimellitic acid have a poisoning effect, i.e., they decrease the rate of oxidation when alkylaromatic compounds are oxidized to aromatic acids. This example is particularly pertinent because during a batch oxidation of pseudocumene there is a buildup of water in the reactor, since this is one of the by-products of the oxidation. Also near the end of the oxidation, there is an increase in the trimellitic acid concentration in the reactor. Both the water and the trimellitic acid will tend to deactivate the catalyst.
Table 2 illustrates the effect of chromium addition to the oxidation of pseudocumene; Table 3 illustrates the effect of molybdenum; and Table 4 illustrates the effect of tungsten. These three elements are in the same chemical family on the periodic table. It can be seen that molybdenum is unique in this chemical family in producing high rates of oxidation during the water and trimellitic acid additions. For example, in Table 3 in Example 9, the rate of oxidation was 0.77 ml dioxygen reacted/min with 20 percent water present in the reactor. However, this rate of oxidation is increased to 3.13 ml dioxygen reacted/min when 0.027 mmole of molybdenum is present (see Example 12 in Table 3). Table 3 further illustrates that increasing molybdenum content in the reactor first results in higher rates of oxidation but eventually decreased rates of oxidation even below the base case. We therefore have a very surprising result in that molybdenum can have either a strong activating effect or a strong poisoning effect depending upon its concentration. There is a optimum amount of molybdenum that gives maximum rates of oxidation.
Table 1 confirms the above results in a pilot plant. Thus, the oxidation in Table 1 was performed in a two-liter titanium-clad autoclave, equipped with a stirrer, internal cooling to control temperature, and inlets so that air and pseudocumene can be added during the oxidation. The vent gases were monitored for dioxygen and carbon dioxide content and were passed through traps to monitor pseudocumene, acetic acid, and water losses. The initial stage of the oxidation was operated semicontinuously where the pseudocumene was continuously pumped into the reactor at 150 psi air pressure and the temperature was slowly increased from 250.degree. to 320.degree. F. over a 25-minute time period. The oxidation was then completed by slowly increasing the pressure from 150 to 500 psi and the reactor temperature was slowly increased from 320.degree. to 425.degree. F. The reaction was terminated when the vent oxygen exceeded 18 percent.
Table 1 clearly illustrates the beneficial effect of molybdenum addition. In Table 1, Example 1 is a base case where no molybdenum was added. The desired trimellitic acid yield is only 59.6 percent. The pseudocumene was only partially oxidized to trimellitic acid as illustrated by the relatively high yield of intermediates. In the succeeding examples, increasing amounts of molybdenum were added into the reactor. The trimellitic yield increased from 59 percent with no molybdenum present to a maximum of 82 percent in Examples 4 and 5. Additional molybdenum addition in Example 16 resulted in reduced yield due mostly to excessive burning of the feedstock and acetic acid to carbon dioxide.
I have found that molybdenum overcomes the waterpoisoning effect; see examples 10-15, 71, and 73 which contain molybdenum and examples 9, 70, and 72 which do not. Water is the product of the oxidation of any alkylaromatic compound. Molybdenum overcomes water poisoning in the oxidation of any alkylaromatic compound, i.e., toluene, ortho-xylene, meta-xylene, para-xylene, pseudocumene, 1,2,3-trimethylbenzene, 1,3,5-trimethylbenzene, durene, etc. Thus, the presence of molybdenum will result in higher rates of oxidation which, in practice, mean that lower concentrations of catalyst can be used; hence, advantageously, a less expensive and much more efficient oxidation process result.
During the batch oxidation of any hydrocarbon, aromatic acids eventually form and increase in concentration as the experiment proceeds. Tables 2-7 illustrate the effect of spiking an oxidation with various types of aromatic acids. I have shown in Tables 3 and 7 that phthalic acid, trimellitic acid, and hemimellitic acid deactivate the catalyst, but acetic, benzoic, isophthalic, and trimesic acids do not. In general, I found that aromatic acids containing two carboxylic acids ortho to each other on the aromatic ring deactivate the catalyst and decrease the rate of oxidation. Tables 3 and 7 also illustrate that molybdenum has the surprising ability to overcome the poisoning effect of aromatic acids such as phthalic, trimellitic, and hemimellitic. I have found that the oxidation of any alkylaromatic compound containing alkyl groups ortho to each other on the benzene ring benefit from molybdenum addition to the catalyst because aromatic acids will be formed which, in the absence of molybdenum, would poison the catalyst. Thus, for example, the oxidation of ortho-xylene, pseudocumene, 1,2,3-trimethylbenzene, durene, 1,2,3,4-tetramethylbenzene, and 1,2,3,5-tetramethylbenzene is aided by molybdenum addition since aromatic acids containing carboxylic acids ortho to each other are formed during the oxidation. Higher rates of oxidation occur with the presence of molybdenum which, in practice, results in lower catalyst concentrations and a less expensive oxidation process.
In the present invention for the oxidation of pseudocumene, the ratio of molybdenum to total conventional metal oxidation catalysts metals to bromine is in the range of about 0.10 to about 2.0 on a gram-atom basis. In our process in addition to molybdenum, cobalt, manganese, and bromine are utilized. The gram-atom ratio of molybdenum to cobalt is in the range of about 0.001 to about 1.0 and the ratio of cobalt to manganese is in the range of about 0.01 to about 100. My invention is also applicable to oxidations wherein in addition to manganese, zirconium is used in addition to cobalt .
Molybdenum can be added to the reaction in any form soluble in para-xylene, meta-xylene, ortho-xylene, pseudocumene, or 1,2,3-trimethylbenzene when it is being oxidized or in acetic acid when it is being used as reaction solvent. For example, molybdenum (VI) bis(2,4-pentanedionate) oxide can be used with manganese for the oxidation of para-xylene or meta-xylene in the presence of acetic acid or water and each of Mn and Co can be conveniently used as its acetate when para-xylene is oxidized in the presence of acetic acid solvent.
The source of molecular oxygen for the molybdenumenhanced oxidation of this invention can vary in O.sub.2 content from that of air to oxygen gas. Air is the preferred source of molecular oxygen for oxidations conducted at temperatures of 100.degree. C. and above, up to 250.degree. C. The minimum pressure for such oxidations is that pressure which will maintain a substantial liquid phase of para-xylene or meta-xylene and 70-80 percent of the solvent. Suitable solvents include formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, trimethylacetic acid, and caproic acid, or water; the preferred solvent is acetic acid. Suitably, mixtures of water and the C.sub.1 -C.sub.6 aliphatic acids can be utilized. It is essential that the molybdenum catalyst be soluble in the solvent. The solvent can amount to 1-10 parts on a weight basis per part of feedstock. The feedstock and solvent, such as acetic acid, not in the liquid phase because of vaporization by heat of reaction, are advantageously condensed and the condensate returned to the oxidation as a known means for removing heat and thereby temperature-controlling the exothermic oxidation reaction. Such vaporization of para-xylene or meta-xylene reactant and acetic acid solvent is also accompanied by vaporization of lower boiling by-product water.
The benefits to be derived from the use of molybdenum according to the present invention are indicated by results shown with respect to the following illustrative and comparative oxidations using pseudocumene as the methyl-substituted benzene to be oxidized.
After termination of the oxidations, the top of the reactor is removed and the total reactor effluents are collected. The resulting TRE products are submitted for aromatic acid analysis.
Product yields are calculated (and hereinafter reported) in mole percent of product per mole of feedstock charged.